1. Field of the Invention
This invention relates to a control system for internal combustion engines, and more particularly to a control system of this kind, which is adapted to control the operation of an internal combustion engine by monitoring the state of combustion of an air-fuel mixture supplied to the cylinders of the engine and using the detected combustion state as a parameter for controlling the operation of the engine.
2. Prior Art
The state of combustion of the mixture in a cylinder of the engine can be rpresented, when the engine is in a steady operating condition, for example, by a ratio .DELTA.Pmax/Pmaxave (hereinafter referred to as "combustion roughness") which is the ratio of an amount of variation .DELTA.Pmax of the maximum value (combustion pressure value) Pmax of pressure within a cylinder to an average value Pmaxave of the maximum value Pmax. The combustion roughness .DELTA.Pmax/Pmaxave assumes a larger value as the state of combustion is worse.
A control system for internal combustion engines is conventionally known e.g. from Japanese Provisional Patent Publication (Kokai) No. 62-38853, which comprises a combustion roughness detector for detecting the combustion roughness by means of a combustion pressure sensor or a combustion light sensor, an air-fuel ratio sensor, and an engine control unit for controlling the ignition timing, air-fuel ratio, etc, of the engine, wherein if the combustion of a mixture in an engine cylinder becomes worse, a fuel injection amount value, an ignition timing advance value, etc. read from respective maps stored in a ROM within the engine control unit are corrected based on a signal from the combustion roughness detector.
On the other hand, oxygen concentration sensors are widely used, which are arranged in the exhaust systems of internal cumbustion engines for detecting the air-fuel ratio of a mixture supplied to the engine. Further, in recent years, there is also used a linear output type oxygen concentration sensor which is capable of detecting the air-fuel ratio A/F not only in the vicinity of a stoichiometric ratio (A/F=14.7) but also over a wider range (e.g. a range of A/F from 8 to 24).
Further, exhaust gas recirculation systems are also widely used in internal combustion engines, for recirculating exhaust gases to the intake system.
However, the combustion pressure sensor and the combustion light sensor mentioned above are very expensive. Moreover, such a sensor has to be provided for each cylinder, and hence the total cost of the control system including the combustion roughness detector becomes considerably high. Therefore, the proposed system cannot be employed in engines in general such as engines for ordinary passenger cars, excepting engines for special applications.
Further, when the engine is operating in a vehicle-cruising condition or an idling condition, which frequently occurs in actual traveling of a vehicle on which the engine is installed, load on the engine is low, and hence the combustion pressure and the combustion light are so weak that it is impossible to accurately detect the combustion roughness by means of the combustion pressure sensor or the combustion light sensor. In particular, when the engine is in a high rotational speed and low load condition, it is quite impossible to obtain reliable results of detection for achieving the intended control of the engine.
Further, in detecting the actual ignition timing and comparing a detected value thereof with a command value supplied to an ignition timing control system for optimum ignition timing control, the combustion pressure sensor is unable to detect the actual ignition timing, which necessitates using both the combustion pressure sensor and the combustion light sensor.
Further, the combustion light sensor, even if formed of a quartz glass optical fiber, has a low durability such that an output therefrom decreases due to aging or deposition of carbon and combustion products on the sensor. Therefore, the combustion light sensor cannot be reliably used for engine control in view of its short service life.
Further, since the conventional linear output type oxygen concentration sensor is complicated in construction and hence expensive, the use of the sesor forms a significant factor for increasing the cost, particularly for detecting the air-fuel ratio of a mixture supplied to each cylinder. Therefore, there is a strong demand for an inexpensive air-fuel ratio sensor having a simpler construction.
Further, the exhaust gas recirculation system is required to control the amount or rate of recirculation of exhaust gases to values suitable for operting conditions of the engine. However, conventional recirculation amount sensors such as an EGR control valve lift sensor do not exhibit fully satisfactory accuracy of detection of an actual amount or rate of exhaust gas recirculation, so that the accuracy of control of the amount or rate of exhaust gas recirculation remains to be improved.